A vehicle may contain safety restraint actuators which are activated in response to a vehicle crash for purposes of mitigating occupant injury. Examples of such restraint actuators include air bags, seat belt pretensioners, and deployable knee bolsters. These vehicle crashes may occur over a wide range of directions relative to the longitudinal axis of the vehicle, and the effectiveness of some restraint actuators can be directionally dependent. A particular air bag restraint system may provide the best occupant protection for collisions in one particular direction along the restraint system axis, and diminishing levels of protection as collision angles depart from the preferred direction. For example, a frontal air bag system provides the best protection for collisions which are directed along the longitudinal axis of the vehicle, while also providing protection but perhaps at a lesser degree for angular or offset crashes, with collision angle measured relative to the longitudinal axis of the vehicle. For collision angles less than 45 degrees in magnitude, the crash is primarily front directed, while for collision angles between 45 and 135 degrees in magnitude, the crash is primarily side directed, while for crashes between 135 and 180 degrees in magnitude, the crash is primarily rear directed.
Both frontal and side-impact air bag systems are well known in the art, and each system is preferably only activated for collisions within its respective range of collision angles. For example, a frontal air bag system might preferably not be activated during a side impact, and a side-impact air bag system might preferably not be activated during a frontal impact. Each such system would have an associated range of angles for which the system is preferentially deployed in the event of a crash for which the occupant might otherwise be injured.
The particular safety restraint actuator(s) which are preferably activated for a given range of crashes is referred to herein as a safety restraint system, whereby a given vehicle may contain a plurality of such safety restraint systems. For each safety restraint system in a given vehicle there is an associated set of crashes of various severity levels which are so directed as to require the activation of the safety restraint system in order to mitigate occupant injury. For the remaining crashes, the restraint system is preferentially not activated so as to minimize the risk of restraint induced injury to the occupant or to avoid unnecessary repair costs associated with the activation of the restraint system.
A safety restraint system is activated by a crash discrimination system which senses the acceleration associated with the crash and determines from the acceleration-time waveform if and when to send an activation signal to the safety restraint system. For an air bag system, this activation signal generally comprises a current of sufficient magnitude and duration to initiate an ignitor which in turn ignites the gas generant composition in an inflator to generate the gases necessary to fill the air bag. The crash discrimination system generally has a restraint sensing axis aligned with the associated restraint system axis. For example, for a frontal air bag system, the restraint system axis and the restraint sensing axis are both aligned with the longitudinal axis of the vehicle, whereas for a side-impact air bag system, the both the restraint system axis and the restraint sensing axis are perpendicular to the longitudinal axis of the vehicle. Generally, acceleration components directed along the restraint sensing axis determine the activation of the associated restraint system, although off-axis components of acceleration can sometimes be interpreted as axial components, especially if the sensor associated with the crash discrimination system is rotated in the course of the crash because of structural deformation of the vehicle.
A crash discrimination system must discriminate between crash conditions requiring restraint system activation--"ON" conditions,--and crash conditions for which the restraint system is preferentially not activated--"OFF" conditions. The borderline between these two conditions is referred as a threshold condition. Those crash conditions near the threshold for which the restraint system is preferentially not activated are referred as "threshold-OFF" conditions (e.g. 8 MPH), while those crash conditions near the threshold for which the restraint system is preferentially activated are referred as "threshold-ON" conditions.
One set of known crash discrimination systems utilizes a plurality of mechanical discrimination sensors positioned and mounted in various locations within the vehicle crush zone or the engine compartment. Each mechanical discrimination sensor generally has a characteristic damping level, which when increased, or over damped, causes the sensor to behave more like a delta-velocity switch; which when decreased, or under damped, causes the sensor to behave more like an acceleration switch. In order to prevent borderline crashes, i.e. "threshold-OFF" conditions, from activating the safety restraint system, mechanical discrimination sensors are generally overdamped, having a delta-velocity threshold in the range of 10-12 MPH, so as to prevent "threshold-OFF" conditions from activating the safety restraint system but with the associated disadvantage that the corresponding "threshold-ON" performance is variable. Generally mechanical discrimination sensors operate by closing a mechanical switch in response to an acceleration signal.
In operation, any one of the plurality of mechanical discrimination sensors can activate the associated safety restraint system. Also, a safing sensor is generally placed in series with the safety restraint system to improve the noise immunity of the system, whereby to activate the safety restraint system, both any one of the mechanical discriminating sensors must be ON, and the safing sensor must be ON, where ON refers to the condition where the sensing characteristic of the sensor has exceeded its associated threshold level. In other words, the activation of the safety restraint system is given by the logical AND combination of the safing sensor with the logical OR combination of the plurality of mechanical discrimination sensors. Safing sensors typically are simply mechanical acceleration switches with a relatively low switching threshold (e.g. 1-2 G's) which is not suitable for crash discrimination because occupants could be harmed by the deployment of an air bag restraint system which might not otherwise be needed to mitigate occupant injury.
Another known crash discrimination system utilizes a single point discriminating crash sensor comprising an electronic control module incorporating an accelerometer, whereby the electronic control module processes the acceleration waveform measured by the accelerometer and outputs a signal to activate the safety restraint system if selected properties of the acceleration waveform according to a sensing characteristic exceed a specific switching threshold. The sensing characteristic is typically implemented by an algorithm executed by a microprocessor in the electronic control module. This activation signal may take a variety of forms, including but not limited to a voltage level, a current level, or a switch closure. The single point discriminating crash sensor is generally mounted at a location within the vehicle from which an acceleration signal is observable for each crash within the set of crashes for which the associated restraint system should be activated. Examples of single point crash discrimination systems are found in U.S. Pat. Nos. 5,067,745, 5,365,114, 5,396,424, 5,495,414 and 5,587,906.
The prior art teaches accelerometer based crash sensors which are self-testable. U.S. Pat. Nos. 5,387,819, 5,506,454, 5,433,101 and 5,495,414 teach the use accelerometers which sense the capacitance of a moveable electrode, whereby the sensing elements may be self-tested with electrostatic deflection. U.S. Pat. Nos. 4,950,914 and 5,428,340 teach the use of piezoelectric sensing elements which are tested by use of a counter piezoelectric effect. U.S. Pat. Nos. 5,375,468 and 5,377,523 teach the use of a piezoelectric accelerometer coupled to a vibrator. U.S. Pat. No. 4,950,915 teaches the use of a piezoelectric sensing element which is tested with acoustic energy. U.S. Pat. No. 5,440,913 teaches the use of dual accelerometers which are continuously tested under normal driving conditions.
The prior art teaches the used of plural redundant accelerometers on a common axis for improved reliability, wherein both accelerometer signals are processed and compared, and if the separate signals from the individual accelerometers are not equal within a certain tolerance the signals are rejected. This known redundant scheme only works along one axis of acceleration. U.S. Pat. Nos. 5,182,459 and 5,363,303 teaches the use of dual piezoelectric accelerometers which are each testable. U.S. Pat. Nos. 5,389,822 and 5,083,276 teach the AND combination of two acceleration sensors installed at approximately the same location. Plural redundant accelerometers sense acceleration along only a single axis. Therefore in systems requiring omnidirectional acceleration sensing and thereby incorporating plural sensing axes, this safing arrangement requires 2 accelerometers for each sensing axis.
A single point discriminating crash sensor may incorporate a safing sensor for improved reliability and protection against erroneous acceleration measurements. An electromechanical switch or safing sensor is physically aligned with the axis of the accelerometer and closes once a certain deceleration threshold is reached. An algorithm processes the vehicle acceleration measurement. The algorithm processor in turn controls a silicon switch which is configured in an AND relationship with the electromechanical switch. Therefore, the electromechanical switch closure must agree in time with the acceleration signal controlled closure in order for the AND requirement to be met. The electromechanical scheme only works along one axis of acceleration. U.S. Pat. No. 5,261,694 teaches that the safing sensor can be reconfigured as a crash discriminating sensor in the event that the single point discriminating crash sensor otherwise fails, whereby this reconfigurable safing sensor is co-located in a common housing with the accelerometer based discriminating crash sensor. U.S. Pat. No. 5,416,360 teaches the combination of a mechanical crash sensor with an electronic crash sensor for improved reliability.
The disadvantage of electromechanical safing sensors for protecting against erroneous acceleration measurements are as follows:
a) Since multiple switches in known systems are configured in an "AND" fashion, both switches must be calibrated to close at the same time over the vehicle life under all environmental conditions.
b) Electromechanical sensors are large and costly.
c) Electromechanical switches can be rendered non-functional if the switches do not close in correct order.
d) Electromechanical sensors have undetectable failure modes which may result in failure to switch timely or failure to switch at all.
e) Electromechanical switch sensors only work on one axis of acceleration. Therefore, if multiple axes of acceleration are required to be "sated", this system requires a separate sensor for every direction of acceleration measurement.
The prior art teaches the control of activation of an air bag system on the basis of collision direction. U.S. Pat. No. 5,609,358 discloses a system incorporating a combination of mechanical crash sensor and accelerometer based crash sensor for detecting collision direction and magnitude upon which decisions are made to either deploy or inhibit deployment of associated plural air bag systems. U.S. Pat. Nos. 4,836,024 and 5,173,614 teach a pair of accelerometers which are angularly displaced left and right of the vehicle longitudinal axis to improve the response characteristic and to determine the impact direction. U.S. Pat. Nos. 5,202,831, 5,234,228 and 5,620,203 teach a combination of longitudinal and lateral crash sensors for detecting crash direction. However, since the axes of the separate accelerometers are skewed with respect to one another, these prior art systems do not provide the redundancy necessary for validation of the separate signals.
The prior art teaches the measurement of acceleration with a plurality of greater than two accelerometers which are skewed relative to one another. U.S. Pat. No. 5,547,149 discloses a system of three strings of accelerometers incorporated in an aircraft safety restraint system. Each string of accelerometers is skewed relative to one another and comprises a plurality of acceleration activated sensors for improved reliability. However, there is no teaching of the utilization of accelerometers from distinct accelerometer strings for purposes of either determining the basis components of the associated acceleration vector, or for improving reliability by testing for self-consistency of acceleration measurements from separate directions.
The prior art teaches redundant accelerometer and gyro elements in strap down navigation units for improved reliability and fault tolerance. In U.S. Pat. No. 4,914,598 a plurality of at least four gyro units and at least four accelerometers skewed relative to one another and arranged with respective axes extending radially with respect to a common reference axis an located on a notional cone the axis of which coincides with the common reference axis. In U.S. Pat. No. 5,297,052 a similar plurality of gyro units and accelerometers is arranged with axes aligned with perpendiculars to six of twelve faces of a regular dodecahedron. Faulty gyro units and accelerometers are identified from parity relationships and using voting circuits. Redundant processors further improve reliability.